Sunburst platy
Sunset platy
They don't seem to appear together on supplier's lists, so they are likely just different names for the same fish with the red back half, trailing off into the tail.
There are also common variations of both:
tuxedo - black in the body
mickey mouse - two black dots on the tail resembling...
neon - more color throughout the body
variatus - crossed to a swordtail, or a generic description for unknown history
wag - black edge on the tail
'nuff said?
/Kris